1. Field of the Invention
The present invention relates to a toner for developing a latent electrostatic image, a full-color toner kit for developing a latent electrostatic image, a developer containing the toner for developing a latent electrostatic image, an image-forming process using the developer, a developer-container which contains the developer, an image-forming apparatus including the developer-container, and an image-forming process cartridge.
2. Description of the Related Art
An image-forming process according to electrostatic developing steps and electrostatic printing steps typically includes a developing step for uniformly charging a photoconductive insulative layer, irradiating the insulative layer with radiation, scattering charges on exposed portions to thereby form a latent electrostatic image, and supplying a toner with fine particles to the latent electrostatic image to thereby visualize the image; a transferring step of transferring the visualized image onto a transfer material such as paper; and an image-fixing step of fixing the image by heating and/or pressurizing, generally using a heat roller. Such developers for developing a latent electrostatic image formed on a surface of a latent electrostatic image support include double-component developers containing a carrier and a toner, and single-component developers (magnetic toners and non-magnetic toners) which do not require a carrier. An ordinary full-color image forming apparatus has functions in which toner images with different colors formed on a photoconductor are sequentially transferred onto an intermediate transfer and are temporarily held thereon. Thereafter, the images are transferred onto a transfer material at once.
Toners for developing an electrostatic image and for printing an electrostatic image mainly comprise a binder resin and a coloring agent and may further comprise a charge control agent, an offset-preventing agent, and, if necessary, may comprise other additives. The toners are required to have various capabilities and properties in each of the steps. For example, to allow a toner to be disposed onto a latent electrostatic image in the developing step, the toners and the binder resin for the toner are required to maintain an appropriate charge amount suitable for use in copying machines or printers, regardless of temperature, humidity, and other conditions. In the fixing step using a heat roller, the toners are required to have satisfactory anti-offset performance so as not to adhere to a heat roller heated to about 100° C. to 230° C. and high image-fixing properties to paper. In addition, the toners are required to have satisfactory blocking resistance, so as not to induce blocking while being stored in a copier.
Various attempts have been made in the techniques for developing a latent electrostatic image so as to furthermore improve image quality. Of those techniques, downsized and spherical toners are believed to be very effective to improve image quality. However, such downsized and spherical toners have deteriorated charging stability and cause scattering of toner particles, where toner particles scatter from a developing unit to inner walls of the apparatus. The scattering of toner particles significantly occurs at high temperature and in high humidity.
Under these circumstances, demands have been made on image formation procedures at a higher speed in color copiers and color printers. To form images at a higher speed, a “tandem system” is effective (as disclosed in Japanese Patent Application Laid-Open (JP-A) No. 05-341617). In the “tandem system,” images formed by an image-forming unit are sequentially transferred and superimposed onto a single transfer paper (transfer material) transported by a transfer belt to thereby form a full-color composite image on the transfer paper (transfer material). Such a color image forming apparatus according to the tandem system accepts a wide variety of transfer papers (transfer materials), can form full-color images with high quality at a high speed. In particular, the apparatus can form full-color images at a higher speed than conventional color image forming apparatus which employs the other systems.
Another attempt has been made to form images at a high speed, at the same time as to attain high image quality using a spherical toner. If an apparatus according to this system is operated at a higher speed, the toner is required to pass through the developing unit in a shorter time. A toner for use herein must therefore be stirred at a higher speed at a higher torque in a charging procedure and developing procedure, so as to achieve a similar developing capability to the conventional developing capability. As a result, the toner may frequently contain weakly charged particles and inversely charged particles. Accordingly, the toner is likely to cause scattering of toner particles from the developing unit.
To improve flowability and charging properties of toners, “external additives” such as metal oxide particles and other inorganic powder are added to the toner particles. To modify hydrophobicity, charging properties, and other properties of the surface on the inorganic powders, the surface of the inorganic powders is treated with a specific silane coupling agent, a titanate coupling agent, silicone oil, or organic acid or the like, or is covered with a specific resin. Examples of the inorganic powder include powder of silicon dioxide (silica), of titanium dioxide (titania), aluminum oxide, zinc oxide, magnesium oxide, cerium oxide, iron oxide, copper oxide, tin oxide, and the like.
Of these, hydrophobic silica fine particles or titanium oxide fine particles are often used. Such hydrophobic silica in fine particles or titanium oxide in fine particles are prepared by allowing fine particles of the silica or the titanium oxide to react with an organosilicon compound such as dimethyldichlorosilane, hexamethyldisilazane, silicone oil or the like to substitute a silanol group on the surface of fine particles with an organic group.
Of these hydrophobing agents, silicone oil has sufficient hydrophobicity and enables a toner containing the silicone oil to exhibit satisfactory transfer properties, due to its low surface energy. Japanese Patent Application Publication (JP-B) No. 07-3600 and Japanese Patent No. 2568244 states the degree of hydrophobicity of silica treated with silicone oil. JP-A No. 07-271087 and JP-A No. 08-29598 state the amount of silicone oil or the carbon content in additives. The silicone oil content and the degree of hydrophobicity are, as disclosed in the JP-A Nos. 07-271087 and 08-29598, sufficient to turn treated inorganic fine particles to be hydrophobic and to ensure stable charging properties of the developer at high humidity.
However, no positive attempt has been made to reduce adhesion of a developer to members to be in contact with the developer utilizing such low surface energy of the silicone oil.
Such members include a contact charging device, a developer-bearing member (sleeve), a doctor blade, a carrier, a latent electrostatic image support (photoconductor), and an intermediate transfer. In particular, toner deposition on the background of images, and dropout after transfer (portions where the developer is not transferred) in edges or centers of characters, lines, and dots in images occur due to strong deposition of the developer to the photoconductor. In addition, when the transfer member has large depressions and protrusions, the image cannot satisfactorily be transferred to the depressions, thus inviting white patches. Simple control of the amount of the silicone oil or the degree of hydrophobicity are insufficient to solve these problems. JP-A No. 11-212299 discloses inorganic fine particles containing a specific amount of silicone oil as a liquid component. However, the use of the silicone oil in the specified amount does not satisfy the above requirements.
The toner for developing a latent electrostatic image must be charged uniformly and stably. If not, the toner causes toner deposition on the background of images or non-uniform image density to thereby deteriorate image quality. A developing unit is downsized, as an image-forming apparatus has been downsized. Rapid charge rise for a toner thereby increases in its importance to obtain high image quality in such a downsized developing unit. To satisfy these requirements, various proposals have been made. For example, to improve charging properties of a toner for developing a latent electrostatic image by adding additives, JP-A No. 03-294864 discloses a non-magnetic single-component developer comprising an inorganic powder treated with silicone oil; JP-A No. 04-204665 discloses a magnetic single-component developer in which an additive covers 3% to 30% of a toner; and JP-A No. 04-335357 discloses an electrostatic developer comprising a toner and an external additive, in which toner has fine particles with a BET specific surface area of 5 m2/g to 100 m2/g fixed on its surface, and which external additive is particles having a specific surface area 1.2 times or more of that of the fine particles fixed on the toner. JP-A No. 07-43930 discloses a developer using a non-magnetic single-component toner including hydrophobic silica fine particles and specific hydrophobic titanium oxide; and JP-A No. 08-202071 discloses a developer containing a toner additive comprising organic-inorganic composite particles having an organic polymer skeleton and a polysiloxane skeleton.
However, even these techniques cannot sufficiently attain uniform charging and good rapid charge rise for a toner. These techniques are not sufficient in stability in surroundings of toner charge, particularly in stability of toner charge with respect to high humidity. Most of these techniques employ an additive having improved hydrophobicity as a result of a surface treatment of oxide particles. The use of such an additive, however, shows deterioration of the toner due to a change in a composition of the additive over a time for operating, although the toner exhibits a desired stable charging at early stages. The composite particles prepared by a liquid phase process as disclosed in JP-A No. 08-202071 may not have sufficient hydrophobicity and may exhibit varying hydrophobicity with time, due to a mediating substance remained inside the particles.
Binder resins for use in toners are required to have transparency, insulating properties, water resistance, fluidity as a powder, mechanical strength, glossiness, thermoplasticity, grindability, and the like. Under these requirements, polystyrenes, styrene-acrylic copolymers, polyester resins, and epoxy resins are generally used as the binder resins. Among them, styrenic resins are widely used for their satisfactory grindability, water resistance, and fluidity. However, when a photocopy obtained by using a toner containing a styrenic resin is stored in a paper holder made of a vinyl chloride resin sheet, an image bearing surface of the copy is left in intimate contact with the sheet. A plasticizer contained in the vinyl chloride resin sheet then migrates into and plasticizes the fixed toner image to thereby allow the toner image to adhere to the sheet. When the photocopy is removed from the sheet, part or whole of the toner image is peeled off from the photocopy and causes toner adhesion on the sheet. This problem also occurs in a toner containing a polyester resin.
To avoid migration of the toner to such a vinyl chloride resin sheet, JP-A No. 60-263951 and JP-A No. 61-24025 propose blending of an epoxy resin with a styrenic resin or polyester resin, since such an epoxy resin is not plasticized by a plasticizer for vinyl chloride resins.
However, when the blended resin is used for color toners, the resulting toner cannot satisfy all of the requirements in anti-offset performance, resistance to curling of fixed images, glossiness, colorability, transparency, and color reproducibility. For example, if a color toner image has insufficient glossiness, it is seen as a weak image. Conventional epoxy resins and acetylated modified epoxy resins, proposed in JP-A No. 61-235852, do not satisfy all of the requirements.
A possible solution to these problems is using an epoxy resin alone. However, such epoxy resins are reactive to amines. The epoxy resins are generally used as curable resins having satisfactory mechanical strength and chemical resistance. These properties are derived from their crosslinked structure formed as a result of a reaction between an epoxy group and a curing agent. Such curing agents are roughly classified into amine curing agents and organic acid anhydride curing agents. Naturally, an epoxy resin for use in a toner for developing a latent electrostatic image is used as a thermosetting resin. However, some dyes, pigments, and charge control agents to be kneaded with the resin to manufacture a toner are amine agents and invite a crosslinking reaction during kneading. The resulting crosslinked article cannot be used as a toner. In addition, the chemical activity of the epoxy group may potentially induce biochemical toxicity such as skin irritation, which must be avoided.
In addition, the epoxy group has hydrophilicity and the toner markedly absorbs water at high temperature and in high humidity. The epoxy group thus causes a decrease in charge, toner deposition on the background of images, and insufficient cleaning. The epoxy resin also shows insufficient charging stability.
Regular toners each comprise a binder resin, a coloring agent, a charge control agent, and other additives to be added according to necessity. Such coloring agents include various dyes and pigments, and some of them have charge control properties and thereby play a role both as a coloring agent and a charge control agent. Such toners having the above composition are generally prepared using a variety of resins as the binder resin. These toners have a problem that the dye or pigment, the charge control agent, and other additives are insufficiently dispersed. The dye or pigment and the charge control agent are generally kneaded with the binder resin in a heat roll mill and must be uniformly dispersed in the binder resin. However, it is difficult to disperse these components uniformly. If the dye or pigment as the coloring agent is not sufficiently dispersed, the toner may exhibit insufficient color development and decreased colorability (degree of coloring). If the charge control agent is not sufficiently dispersed, charges distribute non-uniformly, thus inviting various defects or failures such as charging failure, toner deposition on the background of images, scattering of toner particles, insufficient image density, fuzzing, and insufficient cleaning. JP-A No. 61-219051 discloses a toner using an ester-modified epoxy resin modified with ε-caprolactone as a binder resin. The epoxy resin used herein is modified in a high magnitude of 15% by weight to 90% by weight, and the resulting toner has an excessively low softening point and excessively high glossiness, although it has improved resistance to vinyl chloride resins and fluidity.
JP-A No. 52-86334 discloses an epoxy resin having positive charges prepared by allowing a terminal epoxy group of a prepared epoxy resin to react with an aliphatic primary or secondary amine. However, the epoxy group may crosslink with amine as described above, and the resulting resin may not be used as a toner. JP-A No. 52-156632 discloses that one or both of terminal epoxy groups of an epoxy resin are allowed to react with alcohol, phenol, a Grignard reagent, an organic acid sodium acetylide, or an alkyl chloride. However, a residual epoxy group, if any, may invite problems such as reactivity with amines, toxicity, and hydrophilicity. In addition, some of the aforementioned reaction products are hydrophilic, affect charging properties, or affect grindability in the preparation of toners, and thereby are not always effective to satisfy all of the requirements.
JP-A No. 01-267560 discloses a modified epoxy resin prepared by allowing both terminal epoxy groups of an epoxy resin to react with a monovalent compound having an active hydrogen and esterifying the reaction product with a monocarboxylic acid, an ester derivative or a lactone derivative thereof. The resulting epoxy resin does not exhibit sufficiently improved resistance to curling in image-fixing, although problems in the reactivity, toxicity and hydrophilicity of the epoxy resin are solved.
Solvents such as xylene or the like are often used in preparation of epoxy resins or polyol resins as disclosed in JP-A No. 11-189646. These solvents and unreacted residual monomers such as bisphenol A remain in a significantly large amount in the produced resins and consequently in toners using the resins.
Certain toners using a dye as a coloring agent are disclosed, for example, in JP-A No. 57-130043 and JP-A No. 57-130044. However, these toners using a dye as a coloring agent have insufficient light fastness and undergo discoloring or fading when they are left under direct radiation, although the toners can yield sharp color images with high transparency and good color development.
Toners using a pigment as a coloring agent are disclosed, for example, in JP-A No. 49-46951 and JP-A No. 52-17023. However, the color toners using a pigment as a coloring agent have insufficient colorability (color development) and insufficient transparency due to poor dispersibility of the pigment into a binder resin, although having high light fastness.
To improve dispersibility of a pigment to a binder resin, the following techniques have been proposed.
(1) JP-A No. 62-280755 discloses a technique in which a polyester resin (resin A) is used as a binder resin, a pigment is covered with another polyester resin (resin B) having a molecular weight higher than the resin A in advance, and the covered pigment is dispersed into the resin A to thereby manufacture a color toner.
(2) JP-A No. 02-66561 discloses a color toner comprising a binder resin and a treated pigment dispersed in the binder resin, in which the treated pigment is obtained by melting and kneading a resin and a pigment resin, the pigment resin has a weight-average molecular weight lower than the binder resin, and the binder resin has a weight-average molecular weight of 100000 or more.
(3) JP-A No. 09-101632 discloses a technique for manufacturing a color toner, in which a mixture of a binder resin and a pigment is kneaded with an organic solvent at a temperature lower than a melting temperature of the binder resin in a first kneading step, and the resulting kneaded product is heated, melted and further kneaded with another portion of the binder resin and a charge control agent in a second kneading step.
(4) JP-A No. 04-39671 discloses a toner comprising a binder resin having a weight-average molecular weight of 40000 or less and a coloring agent containing a flushing pigment prepared by using the binder resin.
(5) JP-A No. 04-230770 discloses a technique for preparing a toner, which comprises mixing a solvent with a first binder resin soluble in the solvent and a coloring agent insoluble in the solvent; dispersing particles of the coloring agent into the binder resin at a temperature of 50° C. to 100° C. under a pressure (under a load) and under the application of shear force; removing the solvent to thereby manufacture a colored binder resin composition having dispersed particles of the coloring agent; and heating, melting, and further kneading the binder resin composition with another binder resin and a charge control agent in a second kneading step to thereby manufacture a toner.
However, even according to the techniques (1) and (2), the pigment is not sufficiently dispersed and the resulting toners have insufficient colorability and transparency.
Each of the techniques (3), (4), and (5) exhibits improved dispersibility of the pigment, but employs a solvent. Because of the solvent, the resulting products or toners still contain the solvent in a very slight amount, even though it is supposed to be removed. The inventors of the present invention have found that such a residual solvent in a toner decreases the charge of the toner under special conditions such as high temperature and causes scattering of toner particles in a developing unit. The scattering of toner particles adversely affects the maintainability of the apparatus, and the scattered toner particles adhere to a non-printed portion.
Japanese Patent No. 2992924 and Japanese Patent No. 3047310 disclose toners containing a coloring agent having a specific particle diameter. These toners, however, have insufficiently improved color transparency, color development, and light fastness, although having sufficient colorability. In particular, they cannot avoid scattering of toner particles at high temperature and in high humidity and toner deposition on the background of images at low temperature and in low humidity. JP-A No. 2001-228653 discloses a toner containing a coloring agent having a specific particle diameter distribution, but this toner has insufficient light fastness, since particles having smaller particle diameters are not taken into account.
Such toners are generally produced by a process comprising the steps of mixing all materials at once, heating, melting, and dispersing the resulting mixture to yield a homogenous composition, cooling, pulverizing, and classifying the composition to thereby manufacture a toner having a volume-average particle diameter of 6 μm to 10 μm, as disclosed in JP-A No. 01-304467.
Color toners for use in electrostatic development in the formation of color images generally comprises a color dye or pigment dispersed in a binder resin and require more strict performances than those for use in the formation of black images. Specifically, the color toners must have satisfactory color development (colorability), color reproducibility in composite colors, color developing properties, color gradation, sharpness (definition or visibility), optical transparency when used in over head projectors (OFPs), and high light fastness in any environment, in addition to mechanical and electrical stability to external factors such as impact and humidity. A technique to use a dye for a coloring agent can be found in JP-A No 57-130043 and JP-A No. 57-130044. The technique shows excellent transparency, and enables producing a clear and sharp color image with excellent corlability. The technique, however, shows a poor light fastness, and exhibits shade change and/or discoloring, when left in direct sunshine.
Toners after manufactured are exposed to severe conditions such as high temperature and high humidity or low temperature and low humidity while being stored and transported. The toners must therefore have high storage stability with no or little deterioration in charging properties, fluidity, transfer properties, and image-fixing properties without aggregation of toner particles even after storage under those conditions above. However, no effective solution to these requirements has been found.